The present invention relates to spin transfer torque magnetic random access memory (STT-MRAM), and more particularly, to a method for using a composite hard mask for fabricating a magnetic tunnel junction (MTJ) memory element.
Spin transfer torque magnetic random access memory (STT-MRAM) is a new class of non-volatile memory, which can retain the stored information when powered off. An STT-MRAM device normally comprises an array of memory cells, each of which includes at least a magnetic memory element and a selection transistor coupled in series between appropriate electrodes. Upon application of an appropriate write current to the magnetic memory element, the electrical resistance of the magnetic memory element would change accordingly, thereby switching the stored logic in the respective memory cell.
The magnetic memory element typically includes a magnetic reference layer and a magnetic free layer with an insulating tunnel barrier or junction layer interposed therebetween, thereby collectively forming a magnetic tunneling junction (MTJ). The magnetic reference layer has a fixed magnetization direction and may be anti-ferromagnetically exchange coupled to a magnetic pinned layer, which has a fixed but opposite or anti-parallel magnetization direction. Upon the application of an appropriate write current through the MTJ, the magnetization direction of the magnetic free layer can be switched between two directions: parallel and anti-parallel with respect to the magnetization direction of the magnetic reference layer. The insulating tunnel junction layer is normally made of a dielectric material with a thickness ranging from a few to a few tens of angstroms. When the magnetization directions of the magnetic free and reference layers are substantially parallel, electrons polarized by the magnetic reference layer can tunnel through the insulating tunnel junction layer, thereby decreasing the electrical resistivity of the MTJ. Conversely, the electrical resistivity of the MTJ is high when the magnetization directions of the magnetic reference and free layers are substantially anti-parallel. Accordingly, the stored logic in the magnetic memory element can be switched by changing the magnetization direction of the magnetic free layer.
Based on the relative orientation between the magnetic layers and the magnetization directions thereof, an MTJ can be classified into one of two types: in-plane MTJ, the magnetization directions of which lie substantially within planes parallel to the layer plane, or perpendicular MTJ, the magnetization directions of which are substantially perpendicular to the layer plane.
FIGS. 1A and 1B illustrate selected stages of a conventional process for forming an MTJ memory element as viewed from cross sections thereof. FIG. 1A shows a substrate 50 having a bottom electrode layer 52 thereon, a magnetic tunnel junction (MTJ) layer stack 54 formed on top of the bottom electrode layer 52, a capping layer 56 formed on top of the magnetic tunnel junction layer stack 54, and a metal hard mask 58 formed on top of the top electrode layer 56. The bottom electrode layer 52, the MTJ layer stack 54, and the capping layer 56, are then dry etched with the metal hard mask 58 thereon to form a MTJ memory element as illustrated in FIG. 1B. The dry etching process is normally carried out with a plasma etching process that utilizes a reactive gas chemistry, which would react with the side wall of the MTJ stack 54′ to form a damaged layer 60. The damage layer 60 can significantly degrade the tunneling magnetoresistance ratio, especially when the size of the MTJ stack 54′ decreases.
For the foregoing reasons, there is a need for a manufacturing method that can produce MTJ memory elements with minimal damages thereto.